The Altar of Burnt Offering was square, 7.5 feet on each side, and 4.5 feet high. 

The horns of the altar were marked with blood from the Atonement sacrifice (Leviticus 4:34) They were also a place of refuge for those who had offended the king or accidentally killed someone (1 Kings 1:51; 1 Kings 2:28). Taken together, the horns of the altar symbolize the strength of God and His mercy. Here the sacrifice was made to plead for God’s mercy. Here the blood was shed. It was a daily precursor to the sacrifice of Messiah to come – who is the strength of God in mercy for His people.

In Revelation 9:13, John heard a voice from the four horns of the golden altar before God releasing four angels to kill third of mankind as a “plague”, like the plagues in Egypt, to call the rest of mankind to repentance. Unfortunately in Revelation 9:20, the survivors do not heed the call of mercy extended by God.